1. Field of Invention
This invention is directed to catalysts useful in the conversion of carbon oxides and the processes utilized to manufacture and use those catalysts. Specifically, the invention is directed to a copper/zinc/aluminum catalyst useful for low temperature water gas shift reactions and processes useful for its manufacture and use.
2. Prior Art
Synthesis gas (syngas, a mixture of hydrogen gas and carbon monoxide) represents one of the most important feedstocks for the chemical industry. It is used to synthesize basic chemicals, such as methanol or oxyaldehydes, as well as for the production of ammonia and pure hydrogen. However, synthesis gas produced by steam reforming of hydrocarbons is typically not suitable for industrial applications because the syngas produced is relatively carbon monoxide rich and hydrogen poor.
In commercial operations, a water gas shift (WGS) reaction (Eq. 1) is used to convert carbon monoxide to carbon dioxide.
CO+H2O⇄CO2+H2 H=xe2x88x929.84 Kcal/mol at 298xc2x0 K.xe2x80x83xe2x80x83(Eq. 1)
An added benefit of the WGS reaction is that hydrogen is generated concurrently with the carbon monoxide conversion.
The water gas shift reaction is usually carried out in two stages: a high temperature stage, with typical reaction temperatures of about 350-400xc2x0 C., and a low temperature stage, with typical reaction temperatures of about 180-240xc2x0 C. While the lower temperature reactions favor more complete carbon monoxide conversion, the higher temperature reactions allow recovery of the heat of reaction at a sufficient temperature level to generate high pressure steam. For maximum efficiency and economy of operation, many plants contain a high temperature reaction unit for bulk carbon monoxide conversion and heat recovery, and a low temperature reaction unit for final carbon monoxide conversion.
Chromium-promoted iron catalysts are normally used in the first stage high temperature reactions to effect carbon monoxide conversion at temperatures above about 350xc2x0 C. and to reduce the CO content to about 3-4% (see, for example, D. S. Newsom, Catal. Rev., 21, p. 275 (1980)). As is known from the literature, the chromium oxide promoter serves two functions: it enhances catalytic activity and it acts as a heat stabilizerxe2x80x94increasing the heat stability of magnetite, the active form of the catalyst, and preventing unduly rapid deactivation of the catalyst under conditions of technical use.
The commonly used catalysts for the water gas shift reaction at low temperature (xe2x80x9clow temperature shift (LTS) reactionxe2x80x9d) contain copper oxide, zinc oxide and aluminum oxide. Because these catalysts operate at relatively low temperatures, they generate equilibrium carbon monoxide concentrations of less than about 0.3% in the exit gas stream. However, the performance of the catalyst to effect carbon monoxide conversion and the hydrogen yield gradually decrease during normal operations due to deactivation of the catalyst. This deactivation is caused by poisoning, generally from traces of chloride and sulfur compounds in the feed, or sintering from the hydrothermal environment of the reaction. The rate of the hydrothermal deactivation, in particular, is dependent on reaction conditions such as temperature, steam to gas ratio and composition of the feed gas mixture, and is closely dependent on the formulation and manufacturing process for making the catalyst.
A typical low temperature shift catalyst is comprised of from about 30% to about 70% CuO, from about 20% to about 50% ZnO and from about 5% to about 40% Al2O3. The catalyst is usually made from a precursor formed through co-precipitation of metal salts (nitrate, sulfate, or acetate), thermal decomposition of metal complexes, or impregnation of metal salt onto a carrier. Depending on the preparation conditions (pH, temperature, addition rate and composition), the precursor may include one or several of the following mixed hydroxycarbonate phases: (a) malachite Cu2CO3(OH)2, (b) hydrozincite Zn5(CO3)2(OH)6, (c) rosasite (Cu, Zn)2CO3(OH)2, (d) aurichalcite (Cu, Zn)5(CO3)2 (OH)6 and (e) hydrotalcite (Cu, Zn)6Al2(OH)16CO3. The hydrotalcite is generally present in a hydrated form, such as (Cu, Zn)6Al2(OH)16CO3.4H2O. After preparation, the catalyst is washed to remove foreign ions, dried and calcined at an appropriate temperature to form oxides. With appropriate precursors and preparation conditions, a mixed copper/zinc oxide phase rather than segregated cupric oxide and zinc oxide can be formed during calcination at 250-450xc2x0 C. The catalyst must then be reduced with hydrogen at 100-300xc2x0 C. before being put on stream. During reduction, copper oxide in cupric form is reduced to either metallic copper or/and cuprous oxide.
The precursor material is an important factor in the preparation of the copper/zinc/aluminum oxide water gas shift catalyst. For example, it has been reported that xe2x80x9cCuO/ZnO/Al2O3 mixed oxides should contain CuO in a finely dispersed phase in order to obtain WGS catalysts exhibiting superior activity. For a given copper loading, the CuO crystallite size in the mixed oxide depends on the hydrotalcite content in the hydroxycarbonate precursor: the higher the hydrotalcite amount in the precursor, the lower the CuO crystallite size in the resulting mixed oxide.xe2x80x9d (See Ginxc3xa9s, M. J. L., et al. xe2x80x9cActivity and structure-sensitivity of the water-gas shift reaction over Cuxe2x80x94Znxe2x80x94Al mixed oxide catalysts,xe2x80x9d Applied Catalysis A: General 131, pages 283-296, 295 (1995).) In the Ginxc3xa9s, et al, study, the catalyst which displayed the highest activity was obtained from a catalytic precursor crystallized with a single hydrotalcite-like structure (ibid. at page 291).
While many catalyst compositions have been developed for low temperature water gas shift reactions, improved performance of the catalyst is still sought. In addition, enhancements to the catalyst composition are still necessary to overcome the problems experienced by the prior art catalysts.
Accordingly, an objective of the present invention is an improved catalyst for CO conversion that has superior activity and stability.
It is a further object of the present invention to prepare a catalyst for CO conversion, which catalyst exhibits significant hydrogen production over the lifetime of the catalyst.
It is a still further object of the present invention to disclose an improved low temperature water gas shift catalyst comprising copper, zinc and aluminum.
It is a still further object of the invention to disclose an improved catalyst for CO conversion comprising zinc, aluminum and copper, wherein the catalyst is prepared from a hydroxycarbonate precursor having less than about 60% of the catalyst aluminum in the form of a hydrotalcite, the hydrotalcite being defined as (Cu, Zn)6Al2(OH)16CO3.4H2O.
It is a still further object of the invention to disclose an improved LTS catalyst comprising zinc, aluminum and copper, wherein the copper surface area of the catalyst is greater than about 22 m2/g.
It is a still further object of the invention to disclose processes for the use of the improved catalyst.
It is a still further object of the invention to disclose processes for the products of the improved catalysts.
These and other objects can be obtained by the disclosed processes for the preparation and use of a water gas shift catalyst of the invention and the catalysts produced by those processes.
In accordance with the present invention there is provided an improved low temperature shift copper/zinc/aluminum catalyst which is prepared from a hydroxycarbonate precursor having from about 1% to about 60% of the catalyst aluminum intercalated in a hydrotalcite, wherein the hydrotalcite is defined as (Cu, Zn)6Al2(OH)16CO3.4H2O. In an alternative embodiment, the precursor comprises the hydrotalcite having from about 5% to about 45% of the aluminum of the catalyst, and more preferably the hydrotalcite comprises from about 10% to about 45% of the aluminum. In another alternative embodiment, the precursor comprises the hydrotalcite having less than about 90% of the aluminum of the catalyst, and the catalyst has a copper surface area of greater than about 24 m2/g. In another alternative embodiment, up to about 70% of the aluminum is intercalated in the hydrotalcite and the catalyst has a copper surface area of greater than about 22 m2/g.
The invention is also a process for the production of the catalysts prepared from a hydroxycarbonate precursor having a predetermined hydrotalcite content and copper surface area.
The invention is also a process for the conversion of carbon monoxide and water to carbon dioxide and hydrogen at temperatures in the range from about 150xc2x0 C. to about 350xc2x0 C. using the catalysts described above.